The invention relates to micro-machine devices fabricated from silicon-based materials. Specifically, the invention relates to the surface treatment of silicon-based materials to reduce charge build-up and charge migration.
Grating light valves have applications in display, print, optical and electrical technologies. A grating light valve is a device that is capable of constructively and destructively interfering with an incident light source. Exemplary grating light valves and methods for making grating light valves are disclosed in the U.S. Pat. No. 5,311,360, U.S. Pat. No. 5,841,579 and U.S. Pat. No. 5,808,797, issued to Bloom et al., the contents of which are hereby incorporated by reference.
Grating light valve devices are micro-fabricated from Si-based materials using lithographic techniques. Grating light valve devices are configured to have a plurality of reflective ribbons which are moved by applying an operating bias voltage across the ribbons and a coupled substrate structure. By alternating, or switching, the bias voltage the ribbons are alternated between positions for constructive and destructive interfere with an incident light source having a wavelength xcex.
The ribbons of the grating light valves are preferably formed of Si3N4 and the substrate structure is formed of Si or SiO2. The surfaces of the ribbons and the substrate tend to be strongly hydrophilic and, thus, readily adsorb, physisorb, or chemi-adsorb water or moisture. Adsorbed, physisorbed, or chemi-adsorbed water or moisture on the operating surfaces of the ribbons and the substrate facilitates surface charging. Charging refers to the undesirable collection and migration electrical charges on the insulating surfaces of the grating light valve. Adsorbed, physisorbed, or chemi-adsorbed water or moisture is a difficult parameter to control within the manufacturing process of grating light valves and can severely diminish the performance of grating light valves.
One application for grating light valves is in the field of imaging and display devices, wherein one or more grating light valves are used create a pixel of an image or a pixel of an image on a display device. The presence of surface charging on the operating surfaces of grating light valves can perturb or shift the switching bias voltages. Thus, some of the grating light valves within the display device do not shut off, turn on and/or produce the desired intensity when a bias voltage is applied. The result is the undesirable persistence of an image, portions thereof or the complete failure of the device to produce the image.
To help ensure that charging is minimized, grating light valve structure are handled and manufactured in moisture free or near moisture free environments. Further, grating light valve structures are hermetically sealed within a die structure, after manufacturing, to maintain a moisture free environment. Processing and storing grating light valve structures in moisture free environments is time consuming and expensive. Further, the steps required to seal grating light structures within a die structure adds several steps to the fabrication process.
What is needed is a method to produce micro-fabricated grating light valve structures that exhibit reduced surface charging. Further what is needed is grating light valve structures that exhibit reduced surface charging in open air environments with typical humidity levels.
Grating light valves of the instant invention generate the condition for constructive and destructive interference through a plurality of movable ribbons. The movable ribbons provide a first set of reflective surfaces that are movable relative to a second set of reflective surfaces. The second set of reflective surfaces are reflective surfaces on a substrate element or on a second set of ribbons. In operation, an incident light source having a wavelength xcex impinges on the first set of reflective surfaces and the second set of reflective surfaces. The movable ribbons are displaced towards or away from the second set of reflective surfaces by xcex/4, or a multiple thereof. The portion of light that is reflected from the first set of reflective surfaces and the portion of light that is reflected from the second set of reflective surfaces alternate between being in phase and being out of phase. Preferably, the first set of reflective surfaces and the second set of reflective surfaces are either in the same reflective plane or are separated xcex/2 for generating the condition for constrictive interference.
FIG. 1a illustrates a grating light valve with plurality of movable ribbons 100 that are formed in a spatial relationship over a substrate 102. Both the ribbons 100 and the regions of the substrate between the ribbons have reflective surfaces 104. The reflective surface are provided by coating the ribbons 100 and the substrate with any reflective material such as an aluminum or silver. The height difference 103 between the reflective surfaces 104 on the ribbons 100 and the substrate 102 is xcex/2. When light having a wavelength xcex impinges on the compliment of reflective surfaces 104, the portion of light reflected from the surfaces 104 of the ribbons 100 will be in phase with the portion of light reflected from the surfaces 104 of the substrate 102. This is because the portion of light which strikes the surfaces 104 of the substrate 102 will travel a distance xcex/2 further than the portion of light striking the surface 104 of the ribbons 100. Returning, the portion of light that is reflected from the surfaces 104 of the substrate 102 will travel an addition distance xcex/2 further than the portion of light striking the surface 104 of the ribbons 100 , thus allowing the compliment of reflective surfaces 104 to act as a mirror.
Referring to FIG. 1b, in operation the ribbons 100 are displaced toward the substrate 102 by a distance 105 that is equal to xcex/4, or a multiple thereof, in order to switch from the conditions for constructive interference to the conditions for destructive interference. When light having a wavelength xcex impinges on the reflective surfaces 104xe2x80x2 and 104 with the ribbons 100xe2x80x2 in the down position, the portion of light reflected from the surfaces 104xe2x80x2 will be out of phase, or partially out of phase, with the portion of light reflected from the surfaces 104 and the total reflected light will be attenuated. By alternating the ribbon between the positions shown in FIG. 1a and FIG. 1b, the light is modulated.
An alternative construction for a grating light valve is illustrated in the FIGS. 2a-b. Referring to FIG. 2a, the grating light valve has a plurality of ribbons 206 and 207 that are suspended by a distance 205 over a substrate element 200. The ribbons 206 and 207 are provided with a reflective surfaces 204 and 205, respectively. Preferably, the surface 206 of the substrate 202 also are reflective. The first set of ribbons 206 and the second set of ribbons 207 are initially in the same reflective plane in the absence in the applied force. The first set of ribbons 206 and the second set of ribbons 207 are preferably suspended over the substrate by a distance 203 such that the distances between the reflective surfaces of the ribbons 206 and 207 and the reflective surfaces 208 of the substrate 202 are multiples of xcex/2. Accordingly, the portions of light reflected from the surfaces 204 and 205 of the ribbons 206 and 207 and the reflective surface 208 of the substrate 202, with a wavelength xcex will all be in phase. The ribbons 206 and 207 are capable of being displaced relative to each other by a distance corresponding to a multiple of xcex/4 and thus switching between the conditions for consecutive and destructive interference with an incident light source having a wavelength xcex.
In the FIG. 2b, the second set of ribbons 207 are displaced by a distance 203, corresponding to a multiple of xcex/4 of to the position 207xe2x80x2. The portion of the light reflected from the surfaces 205xe2x80x2 of the ribbons 207 will destructively interfere with the portion of the light reflected from the surfaces 204 of the ribbons 206.
FIG. 3 plots and intensity response 307 of a grating light valve to an incident light source with a wavelength xcex when and voltage 308 is applied across a selected set ribbons (active ribbons) and the underlying substrate. From the discussion above, the brightness value will be at a maximum when the ribbons are in the same reflective plane, separated by xcex/2, or a multiple of xcex/2, and brightness will be at a minimum when the ribbons are separated by xcex/4, or a multiple of xcex/4.
The curve 306 illustrates the initial intensity response of a grating light valve to an applied voltages without significant surface charging. The curve 309 illustrates the intensity response for the same grating light valve to an applied voltage after surface charging has occurred. The curves 306 and 309 are offset by a value 310, which can be on the order of several volts. Such shifting in the intensity response curve is undesirable especially in display applications.
In display applications, the response of a grating light valve to an applied voltages is carefully calibrated to achieve a desired intensity level accurately. For example, eight bit voltage drivers subdivide the voltage curve, such as the voltage response curve illustrated in FIG. 3, into 265 grey levels. Clearly, a response curve shift of even a fraction of a Volt will seriously degrade the ability of the device to produce a desire intensity level.
Whether a grating light is constructed according to the principles illustrated in FIGS. 1a-b, FIGS. 2a-b, or any other construction utilizing ribbons moved by applying a bias across the ribbons and the substrate, there is the tendency for the ribbon surfaces and the substrate surfaces to exhibit charging. Charging on the surfaces of the ribbons and the substrate perturbs or the optical response causing the grating light valve to fail. Therefore, there is a need to provide grating light valve constructions which exhibit reduced charging.
According to the present invention, a micro-device which is fabricated from a silicon-based material and has silicon-based surfaces is treated with a pacifying gas to reduce surface charging. Preferably, the micro-device is a grating light valve with a plurality of movable ribbons comprising Si3N4 surfaces coupled to a substrate element comprising SiO2 surfaces, wherein the ribbons alternate between the conditions for constructive and destructive interference with an incident light source having a wavelength xcex by apply the appropriate switching voltages across a selected portion of the ribbons and the substrate.
In accordance with the preferred method of the instant invention a grating light valve structure comprising silicon-based surfaces is placed in a vacuum environment with a pressure of 10xe2x88x926 Torr or less. The grating light valve structure is heated in the vacuum environment to temperatures of at least 250 degrees Celsius for a period of time sufficient to remove residual water or moisture form the surfaces of the structure; preferably 1 hour or more. The grating light valve is then allowed to cool to ambient temperatures and is exposed to a pacifying gas environment. Alternatively, the device is treated with the pacifying gas at elevated temperatures and is then allowed to cool to ambient temperatures. A cycling process of placing the grating light valve in the vacuum environment, heating the grating light valve, exposing the grating light valve to the pacifying gas environment and cooling the grating light valve is performed any number of times to achieve the intended goal of pacifying the surface and reducing charging of the surfaces.
Preferably, the surfaces of the grating light valve are pacified within isolation chamber where a vacuum environment with a pressure of 10xe2x88x927 Torr or less is achieved. Further, it is preferable that the pacifying gas used is substantially dried with a water content of less than 1 ppm. Further it is preferred that the pacifying gas contains a substantial amount of Nitrogen (50% or more) in combination with an noble Group VIII gas, such as Argon or Helium. Alternatively, the pacifying gas is approximately 100% dried Nitrogen.
After the grating light valve is cooled, the grating light valve is hermetically sealed within a die structure and installed in the intended device. Alternatively, the device is directly installed in the intended device and operates in an open air environment.